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10) 8896 



Bureau of Mines Information Circular/1982 




Surface Subsidence Over Longwall Panels 
in the Western United States 

Monitoring Program and Preliminary Results 
at the Deer Creek Mine, Utah 



By Frederick K. Allgaier 



(>^^ 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular/8896 



Surface Subsidence Over Longwall Panels 
in the Western United States 

Monitoring Program and Preliminary Results 
at the Deer Creek Mine, Utah 



By Frederick K. Allgaier 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



^ 



0^ ^ 



This publication has been cataloged as follows: 



Allgaier, F. K. (Frederick K.) 

Surface subsidence over longwall panels in the Western United 
States. Monitoring program and preliminary results at the Deer C'reck 
Mine, I'tah. 

nnformation circular ; 8896) 

Includes bibliographical references. 

Supt. of Docs, no.: I 28.27:8896. 

1. Mine subsidences — Utah. 2. Coal mines and mining — Utah. 1. 
T itie. II. Series: Information circular (United States. Bureau of 
Mines) : 8896. 



TN295,IJ4 ITN3191 H22s 1 622\3341 82-600171 AACR2 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Acknowledgments 2 

The Deer Creek Mine study site 3 

Site selection 3 

Site description 3 

Regional geology 6 

Stratigraphy 7 

Mine plan 9 

Subsidence monitoring program 13 

Monitoring program design 13 

Monument locations 14 

Monument spacing 14 

Monument construction 14 

Monument Installation and survey schedule 15 

Monitoring procedures 17 

Measured surface subsidence 19 

Subsidence development 23 

Data processing 23 

Conclusions 24 

ILLUSTEIATIONS 

1 . Project location map 3 

2. Surface contours over the longwall panels 4 

3 . Surface topography 5 

4. Typical surface features over the longwall panels 6 

5. Regional geologic structures. 7 

6. Regional stratigraphy 8 

7 . Regional faulting 9 

8. Generalized overburden stratigraphy 10 

9. Coal seam stratigraphy 11 

10. Mine plan with local faulting 12 

1 1 . Overburden Isopachs 13 

12. Subsidence monuments 15 

13. Monument Installation with a gas-powered hammer 16 

14. Theodolite with distance meter 18 

15. Target-prism unit 19 

16. Level Instrument and rod 20 

17. Subsidence profiles — panel 5E 21 

18. Subsidence profiles — panel 6E 21 

19. Subsidence contours 22 

20. Subsidence profiles across panels 5E and 6E..... 22 



^^ 



SURFACE SUBSIDENCE OVER LONGWALL PANELS IN THE WESTERN UNITED STATES 
Monitoring Program and Preliminary Results at the Deer Creek Mine, Utah 

By Frederick K, Allgaier^ 



ABSTRACT 

This is the first in a series of progress reports on the longwall sub- 
sidence research program at the Bureau of Mines Denver Research Center. 
As part of this program, the Bureau and the Utah Power and Light Co. are 
cooperating on a study conducted at the Deer Creek Mine, which is 
directed toward developing the capability to estimate the surface sub- 
sidence resulting from longwall mining in a geologic, topographic, and 
mining environment common to coalfields in the Western United States. 

A monitoring network has been established at the Deer Creek Mine to 
measure subsidence over four adjacent longwall panels. To date, two 
panels have been mined. Subsidence began as the first panel was mined 
and continued for 1 year following completion of the panel, during which 
time the adjacent panel was mined. A maximum of 2.7 feet of subsidence 
occurred over the two longwall panels mined at a depth of 1,500 feet. 
Because of the length of time that subsidence continued after mining, 
the final subsidence profiles and angle of draw have not yet been 
determined. 



^Mining engineer, Denver Research Center, Bureau of Mines, Denver, Colo. 



INTRODUCTION 



Population density and surface develop- 
ment over active coal mines in Great 
Britain and other parts of Western Europe 
have dictated for many years that surface 
subsidence and its effects be thoroughly 
understood and that mine engineering 
practices be developed to reduce these 
effects. It is only recently that the 
need to develop a subsidence data base 
representative of U.S. mining conditions 
has been recognized. In addition, long- 
wall mining has only recently begun to 
gain substantial acceptance in the United 
States, particularly in the West. Hence, 
subsidence from longwall mining is still 
a largely unknown and unpredictable 
quantity. 

The public sector and various State and 
Federal Government agencies that own the 
surface over much of the western coal 
deposits have become increasingly con- 
cerned about the effects of subsidence on 
future land use, as well as its impact on 
surface and subsurface hydrology and sur- 
face structures. Consequently, mining 
companies are being required to examine 
more closely the possible surface impacts 
of underground mining. 

A major problem now faced by mine op- 
erators and landowners in the Western 
United States is the lack of actual case- 
history data that document subsidence 
sufficiently for use in estimating sub- 
sidence values and environmental impacts 
for specific properties. Factors such 
as strong, massive sandstone members 
in the overburden, thick and multiple 
seams, deep cover, and extreme variations 
in overburden thickness over a single 
panel due to mountainous topography are 
common in the Western United States and 
can have significant impact on subsidence 
characteristics. 



As mine operators and environmental 
agencies attempt to address subsidence 
issues in the mine planning and permit- 
ting process, the lack of applicable sub- 
sidence information, experience, and pre- 
diction capabilities becomes apparent. 
Bureau of Mines research in subsidence 
from longwall mining is directed toward 
fulfilling the needs of the industry in 
the premining evaluation of surface dam- 
age and in facilitating the permitting 
process. As part of this program, the 
Bureau's Denver Research Center and the 
Utah Power and Light Co. are cooperating 
on a study conducted at the Deer Creek 
Mine in Emery County, Utah. 

The major objectives of the Deer Creek 
study are to (1) measure surface subsi- 
dence caused by longwall mining in the 
Blind Canyon coal seam, (2) determine 
the timing, rate, and areal extent of 
subsidence, (3) establish the final sub- 
sidence profiles, (4) correlate mining 
and geologic variables with measured sub- 
sidence values, (5) evaluate predictive 
capabilities with regard to actual, mea- 
sured subsidence versus theoretical val- 
ues, and (6) determine the effects of 
subsidence on current and potential land 
use. 

This report describes the Deer Creek 
study site, the methods and instruments 
used in subsidence surveys, and the 
status of the monitoring program, and 
includes a preliminary discussion of the 
surface subsidence measured through 1980. 
The final subsidence profiles and a com- 
plete analysis of the subsidence data 
from this site will be contained in a 
future report. Additional reports cover- 
ing similar subsidence investigations at 
other mines are also planned. 



ACKNOWLEDGMENTS 



The Utah Power and Light Co., Mining 
and Exploration Department, provided val- 
uable assistance in conducting this re- 
search. In particular, Don Dewey, Chris 
Shingleton, John Bootle, Jeff McKenzie, 
and Roger Fry have made significant 



contributions to the project. Without 
the access they provided to company prop- 
erty, mine plans, survey data, drill 
logs, and other information relating to 
the Deer Creek Mine, this study could not 
have been conducted. 



The efforts of Bureau of Mines person- 
nel who performed the field work for the 
project are also acknowledged. Laura 



Swatek, in the Bureau's Mine Engineering 
Division, prepared the geologic descrip- 
tion of the study site for this report. 



THE DEER CREEK MINE STUDY SITE 



Site Selection 

The Bureau selected the Deer Creek 
Mine, owned by Utah Power and Light Co., 
as one of the sites for monitoring subsi- 
dence over longwall panels, because it 
contains specific raining and overburden 
features for which few or no subsidence 
field data exist. These features include 
1,500 feet of overburden containing a 
significant percentage of strong sand- 
stone members, an extraction height of 10 
feet, and a lower seam to be longwall 
mined, which will present an opportunity 
to study subsidence from multiple seam 
mining. Also, the timing of the project 
was such that monitoring could begin over 
the first in a series of four adjacent 
longwall panels. This was important 



because one of the questions to be an- 
swered by the study is how much area must 
be mined, in terms of either face advance 
or width over adjacent panels, tefore 
subsidence occurs at the surface. The 
conditions at the Deer Creek Mine are 
somewhat representative of many western 
mines; therefore, the results are ex- 
pected to be applicable to mines in other 
western areas. 

Site Description 

The study site over the Deer Creek Mine 
is located on East Mountain in Emery 
County, Utah, approximately 10 miles west 
of the town of Huntington. The site in- 
cludes parts of sections 14, 15, 22, and 
23;T17S; R7Eon the Mahogany Point 




Scale, miles 



r 



Carbon County 
Emery County 



To Price 




Orangeville 



FIGURE 1. - Project location map. 



and Red Point 7.5-minute U.S. Geological 
Survey quadrangle maps. 2 The project 
site location is illustrated in figure 1. 
Portions of the study site are within the 
boundaries of the Manti-LaSal National 
Forest, and the remainder is controlled 
by Utah Power and Light Co. Approximate- 
ly 500 acres are included in the monitor- 
ing area over four adjacent longwall 
panels. 



The topography over the panels is gen- 
erally rolling, with no outcrops or ver- 
tical faces (figs. 2-3). The maximum 
ground slope in the area is approximately 
45 percent, with a maximum relief of 300 
feet over the entire site. No unusual 
problems due to topography were encoun- 
tered in either the installation or moni- 
toring of the subsidence network. 




900 

'■''■■''' ' 
Scale, feet 

FIGURE 2. - Surface contours over the longwall panels. The subsidence monitoring network, 
as lines of crosses, is discussed in the section "Monument Locations." 



sh 



own 



^U.S. Geological Survey maps N3915-W1 1 107. 5/7. 5 and N3915-W1 1 100/7.5, 1979. 




FIGURE 3. - Surface topography. 



The average surface elevation of the 
study site Is 9,100 feet, and vegetation 
consists mostly of sagebrush, with some 
significant areas of pine and aspen 
(fig. 4). The location of the longwall 
panels Is such that surface vegetation 
over three of the four panels Is entirely 



sagebrush, which allowed clear line 
of sight for the monitoring surveys. 
Approximately one-half of the first panel 
(5E) lies under a wooded area, which 
required cutting and clearing of survey 
sight lines. 




FIGURE 4. - Typical surface features over the longwall panels. 



The elevation of the site is a signifi- 
cant factor in the monitoring program. 
Early fall and late spring snowfalls, 
which can make the site inaccessible from 
October into June, prevent subsidence 
monitoring surveys from being carried out 
at equal intervals throughout the year. 
Therefore, the magnitude and timing of 
any subsidence occurring during the win- 
ter months must be interpolated between 
the last survey performed in the fall and 
first survey in the spring of the next 
year. 

Regional Geology 

The Deer Creek. Mine is located in cen- 
tral Utah on the Wasatch Plateau, a 



broad, linear structure that lies gener- 
ally in a north-south direction (fig. 
5). The strata dip gently westward in 
the eastern part of the plateau because 
of the presence of the west flank of the 
San Rafael Swell. The western part of 
the plateau marks the transition into the 
highly faulted and complex region of the 
Great Basin. 3 

Sedimentary rocks form the majority of 
the stratigraphic sequence within the 

•^Davis, F.D., and H. H. Doelling. Coal 
Drilling at Trail Mountain, North Horn 
Mountain, and Johns Peak Areas, Wasatch 
Plateau, Utah. Utah Geol. and Miner. 
Survey Bull. 112, 1977, p. 4. 




FIGURE 5. - Regional geologic structures. 

Wasatch Plateau region. The sequence 
consists of one limestone unit and alter- 
nating sandstones, silt stones, and mud- 
stones. The two major coalbeds occur 
within the lower portion of the strati- 
graphic section. The rocks are sub- 
divided into seven formations (fig. 6) 
and range in age from early Cretaceous to 
Paleocene.4 

^Spieker, E. M. The Transition Between 
the Colorado Plateaus and the Great Basin 
in Central Utah. Guidebook to the Geol- 
ogy of Utah. No. 4. Utah Geol. Soc. , 
1949, p. 52. 



Faults are a very prominent feature 
in this area (fig. 7), representing a 
transitional zone between the Wasatch 
Plateau and the Great Basin to the west. 5 
These are high-angle, normal faults 
trending nearly north-south. Vertical 
displacements of stratigraphy range from 
0.5 foot to nearly 200 feet. The long- 
wall operations involved in this study 
are being conducted between the Pleasant 
Valley Fault to the west and the Deer 
Creek Fault to the east. There are no 
known faults crossing the longwall 
panels. 

Stratigraphy 

Drill hole records supplied by Utah 
Power and Light Co. were used to deter- 
mine the stratigraphy of the overburden 
above the longwall panels. The gener- 
alized stratigraphic section (fig. 8) 
illustrates that sandstones and inter- 
beds of siltstones, sandstones, and 
mudstones are predominant units. The 
two major sandstone units are the 
Castlegate Sandstone, which is found in 
the lower Price River Formation, and the 
Star Point Sandstone, which is located 
in the lower Blackhawk Formation below 
the Blind Canyon Coal Seam (fig. 6). In 
this area, the Castlegate Sandstone is 
described as a buff to gray, massive, 
fine-grained, well-cemented unit with 
occasional silty bands occurring through- 
out. The Star Point Sandstone is a 
light-gray, fine-grained unit that is 
well sorted and quartzose. The total 
percentage of sandstone in the overburden 
is between 35 and 45; 35 percent repre- 
sents that occurring in thick beds, and 
45 percent includes all thin beds and 
laminations. 

^Work cited in footnote 3. 



System 


Group 


Formation 


Thickness, 
feet 


Description 






Flagstaff Limestone 


100-1,000 


Light-gray to cream 
limestone; thin and 
even-bedded; dense; 










fossil iferous; ledge- 




X 






and cliff-forming. 




(J 

•♦-> 








1- 


to 
re 

3 


North Horn Formation 


900-2,000 


Mostly red-brown, and 
salmon-colored shales; 
varying thicknesses of 
sandstone, freshwater 
limestone and conglom- 
erate; slope-forming. 






Upper Price 


400-800 


Mostly tan and gray. 






River Member 




medium-to coarse-grain- 






c 




ed sandstone and some 






o 

•r- 




gray shale and conglom- 






4-> 

ro 




eratic sandstone; ledge- 






E 
i. 
o 




and slope-forming. 






Ll_ 

s- Castlegate 


150-500 


Light-gray, yellowish- 






> Sandstone 




brown and white, medium- 






S Member 




to coarse-grained sand- 






<u 




stone and conglomeratic 




<U 






sandstone; cliff-form- 


to 


"O 


i~ 




• 


3 


s_ 


a. 




ing. 


o 


a; 


r^-i f*^^^fr\^mA + i/ 






a; 
o 

■•-' 


> 
re 
to 
0) 


Blackhawk Formation 


400-1,000 


Light-to medium-gray 






« 




sandstones; gray to 
black shale; gray silt- 
stones; important coal- 
beds in lower half; 
sandstones weather tan, 
brown, yellowish-brown; 
ledge-and slope-forming. 






Star Point Sandstone 


200-1,000 


Tan, light-gray, and 
white massive sand- 
stones separated by one 
or more shale tongue; 
cliff-forming. 


to OJ 
o .— 


Masuk Shale 


300-1,300 


Light-gray to blue-gray 




u re 
c x: 






sandy marine shale; 




re </5 






thins to west and south; 
slope-forming. 



FIGURE 6. - Regional stratigraphy. (Modified from Utah Geological and Mineral Survey Bulletin 112, 
1977, p. 4.) 



Huntington 




Blackhawk outcrops IvkM 

FIGURE 7. - Regional faulting. 

The Deer Creek Mine produces coal from 
the Blind Canyon Seam in the coal-bearing 
zone of the Blackhawk Formation. The 
Blackhawk Formation consists mainly of 
medium- to fine-grained interbedded sand- 
stones, carbonaceous muds tones, and silt- 
stones. The Blind Canyon Seam, with a 
coal thickness of 14.3 feet in the panel 
area, is described as a hard, dense, 
bright-attrital coal and is ranked as 
high volatile, B-bituminous. Figure 9 
illustrates the general stratigraphy of 
the coal seam, the immediate roof, and 
the immediate floor as determined from 
drill hole records supplied by Utah Power 



and Light Co. The jointing pattern with- 
in the coal seam and surrounding strata 
consists of vertical joints with an aver- 
age trend of N 21° W. The joints are 
very pronounced in the sandstone units 
and tend to be faint to undistinguishable 
within the muds tone units. Spacing of 
the joints is considered moderate, with 
an average spacing of 10 to 20 feet. 

Mine Plan 

The mining plan for the longwall panels 
at the Deer Creek Mine is shown on fig- 
ure 10. It consists of four adjacent 
panels oriented in an east-west direction 
to be retreat mined from the east toward 
the main entries to the west. 

A room-and-pillar section (4E) north of 
the first panel was mined prior to the 
development of the longwall panels. This 
section consisted of 20-foot-wide rooms 
on 100-foot centers with crosscuts on 80- 
foot centers. Pillar mining was not con- 
ducted; however, 1 to 2 feet of floor 
coal was mined on retreat, increasing the 
mining height to 10 feet. A 200-foot 
barrier pillar was left between third 
east section (3E) and fourth east section 
(4E), which includes the tailgate entries 
for the first longwall panel (5E), 

The first panel was developed using a 
three-entry system, and the remaining 
three panels were developed by two en- 
tries on 50-foot centers and crosscuts on 
105-foot centers. 

Mining height for the first two long- 
wall panels averages 10 feet. The length 
of both panels 5E and 6E is 2,500 feet, 
with face lengths of 480 feet and 540 
feet, respectively. 



10 



DEPTH. LiTHOLOGY 
feet 



0- 



> .o. a V'. o 
a .0 g ■ o-e 



50- 



100- 



I50-- 



200-Z 



250- 



■TT — t— r-r-r- 



300- 



350- — 



400- 



450- 



500' 



500- 



550- 



600- 



650 



700- 



750- 



800 



T 



1,100- 



Cas+legate 
Sandstone '.'50- 



850- 



900- 



Castlegate 
Sandstone 



LEGEND 
In+erbeds 



1,000- __ 
l,050-~ 



:'•'•!••] Sandstone 



950-=^?-:^'-'^^ 



|^r£^ Siltstone 

p^£^ Mudstone 
^B Coal 



[f^ Alluvium 



1,300- 



1^50- 



1,200- --^^= = 
1,250- 



1.400- 



1.450- 



1.500- 




Bllnd Canyon 
Coal Seam 



Star Point 
Sandstone 



FIGURE 8. - Generalized overburden stratigraphy. 



11 



DEPTH, 
feet 

1,5 1 - 



1,515- 



1,5 20- 



LITHOLOGY 



DESCRIPTION 



0.8ft mudstone; gray, dense 



0.7ft sandstone; light-gray,fine-grained, moderately sorted 



1.8 ft mudstone with moderate plant remains 



5.7 ft interbedSi principally dense mudstone with 
some fine-grained siltstone 



=:^'^5ft mudstone •, homogeneous, dense, gray 



1,525 



1,530 



1,535 



1,540 




2.6ft carbonaceous mudstone ; black, locally fissile 



14.3ft coal; Blind Canyon Seam, bright-attrital 
hard, dense 



14 ft mudstone 



3.2ft interbeds; si Itstone i light-gray and 
mudstone; gray, dense 



0.5 ft coal 






^0.2ft carbonaceous mudstone-, black, dense 
0.3ft mudstone-, gray, dense 



= i:^^^f=^^^ 2.9ft siltstone; light grey 



'= — -.^z=^^= 0.6ft carbonaceous mudstone •, dark, dense 



1,545 



0.5ft bone coal 



FIGURE 9. - Coal seam stratigraphy. 



200-foot barrier pillar 




DOaaao 

ODODDD 



aOOODDODOaOODDOtlDanDODQ 



^ 



gOOflDDLJoODaOD.p q^^tmn QDDOQDQDaQOOaQOQCaDl 
QDGOODDOOQODOOD aecTion □□□□qdoodQDDQQQOdQOQOC 
aODDOODaOPOOQOODOOaoaoODODDODQDDaDDQOnQDDCiaODC 



(DCiDDOO 
ODDOO 

Dppiaa 



5E longwall panel 



\0oq/ 

Qa 

QD 
DO 



6E longwall panel 



p. QQQr-irjrl PQQDaCJ OOODOQDC3C30 OC3C:3CJC3POt=»C3P aCloC_ 

aaaoa,'' *° 

aaoool 

DDDDO 
DOOOO ^ 

V~in OOOQO P '^'="^'=^'='°'^'^'^t=l'^^ DCSOO C3C300C3C3C3CJOC JQC^ 

DOOODD -,,-1 II I 

aaaoQQ ^^ longwall panel 
_aaaQaQaaDDaDl 
nQOaaaDaac7000a^\,_^ , 

go D o_aaa D a oo dP 

EdSSL ^^ '°"9wall panel 

Qoa aa Q^s 

(JOaoa 
ooaoa 

OOODD 
oQQOa 
aaoaci 
Daoaa 

□ aoDO 

□ □doq 

OOODQ 
nnonn 



O 



> o C3 C7 cyczicaC:^ OO < 




FIGURE 10. - Mine plan with local faulting. 



180" 



100'. 



500 



1,000 



Scale, feet 



The depth of cover over the first two 
panels ranges from a maximum of 1,580 
feet to a minimum of 1,300 feet (fig. 
11). The maximum overburden occurs near 
the center of the panels and decreases » 
toward both ends. The seam dips only 1.3 
degrees to the northwest and therefore 
has little effect on the depth of cover, 
which is controlled almost entirely by 
the surface topography. 

Mining of the first panel began in May 
1979 and was completed in December 1979. 



The second panel was mined between Febru- 
ary and December 1980. 

A 400-foot barrier was left between the 
end of the panels and the main entries to 
the west, Unmined coal remains between 
the longwall panel starting rooms and the 
Deer Creek Fault, approximately 200 feet 
to the east. There has been no previous 
mining either above or below the panels, 
although the Wilberg Mine will subse- 
quently undermine this section of the 
Deer Creek Mine. 



13 




Scale, feet 



FIGURE n. - Overburden isopachs. 
SUBSIDENCE MONITORING PROGRAM 



Monitoring Program Design 

Several factors Influenced the design 
of the subsidence monitoring program at 
the Deer Creek Mine. The most important 
consideration was that the information 
collected would meet the established 
project objectives of determining the 
maximum subsidence, the areal extent of 
subsidence, and the rate at which subsid- 
ence progresses down the panels. In 
addition, final subsidence profiles would 
be developed and analyzed with respect to 



the overburden geology and mine layout 
and then correlated with existing pre- 
dictive techniques. Major items that 
affect the subsidence data being collect- 
ed are surveying accuracy and frequency, 
monument location, in terms of both spac- 
ing and layout over the panel, monument 
construction, and surveying instrumenta- 
tion. Constraints on the monitoring pro- 
gram caused by the short field season and 
personnel limitations affect the number 
and type of surveys performed and thus 
the final project results. 



14 



Monument Locations 

The locations of the subsidence monu- 
ments were established on the basis of 
coordinate survey data supplied by Utah 
Power and Light Co. for points on the 
surface in or near the study site and for 
the location of the longwall panels. 
Both the underground and surface surveys 
are tied to the Utah State plane coordi- 
nate system, which allows direct correla- 
tion between surface and underground 
positions. 

Soil cover in the area ranges from a 
few feet to approximately 20 feet in 
depth. Although the soil is moderately 
rocky, it is of adequate depth to permit 
the monuments to be installed with few 
problems. The only area where monuments 
were difficult to install was at the cen- 
ter of the first panel (5E) on a topo- 
graphic high near the transverse line of 
monuments (fig. 2). Problems in this 
area were solved by changing the monument 
location by a few feet to an area that 
could be penetrated. In no case was a 
monument omitted from a planned location 
because of inability to drive it into 
rocky soil. 

The network layout used for this site 
is shown on figure 2. It consists of 
one line of monuments approximately 
centered over the long axis of each 
panel and several transverse lines 
located at specific positions over the 
panel. This type of monitoring layout 
produces both transverse and longitu- 
dinal subsidence profiles. In addi- 
tion, a diagonal line of monuments on the 
east end of the panels was included to 
provide more data on the angle of draw 
and the interaction of the combined 
subsidence at the corners of the two 
adjacent panels. The transverse line of 
monuments running north-south at the 
center of the panels lies over the area 
of maximum overburden; the transverse 
line over the western end of the panels 
lies over the area of minimum overburden 
thickness; and the transverse monument 
line just east of the panels is over an 
area expected to be affected by the angle 
of draw. 



The location of stable, remote control 
points is governed by topography and veg- 
etation, which affect the line of sight 
to the subsidence monuments. The loca- 
tions of the underground workings also 
dictate the areas that will remain stable 
throughout the life of the project. At 
the Deer Creek site it was not always 
possible to locate control points on sta- 
ble ground because the topography blocked 
lines of sight to the subsidence monu- 
ments. In these instances, control 
points were located over the panels and 
tied to at least two stable points with 
vertical and horizontal surveys. The 
accurate location of these control points 
was established for each survey of the 
monitoring network. 

Monument Spacing 

Monument spacing on the subsidence mon- 
itoring network is 100 feet. This spac- 
ing was felt to be a practical compromise 
between the more accurate determination 
of strain and angle of draw that is pos- 
sible with decreased spacing and the 
increased cost of installing and survey- 
ing the additional points. The 100-foot 
spacing, an average of 0.07 times the 
overburden depth, is somewhat larger than 
the 0.05 recommended by the National Coal 
Board (NCB) Subsidence Engineers' Hand- 
book. 6 However, the NCB acknowledges 
that there is a practical limit to reduc- 
ing monument spacing and that further 
research is needed before the optimum 
spacing can be defined. As part of the 
continuing work at the Deer Creek site, 
selected portions of the fourth longwall 
panel will be monumented with 50-foot 
spacing, and the resulting angles of draw 
and direct strain measurements will be 
compared with similar measurements from 
the panels with monuments on 100-foot 
centers. 

Monument Construction 

Two different types of monuments have 
been used on the project (fig. 12). One 

^National Coal Board, Great Britain. 
Subsidence Engineers' Handbook. 1975, 
p. 33. 



15 



6 10 15 

LuxUi-LuliaxJ 

Scale, cm 



FIGURE 12. - Subsidence monuments. 



type consists of 1-1/2-inch pipe cut to 
length, with a bevel on one end to facil- 
itate installation. The other type is 
1-inch steel rod with a machined point. 
Both types of monuments were driven into 
the ground to a depth of 3 to 5 feet with 
either a gas-powered hammer (fig. 13) or 
a sledge hammer. Approximately 6 inches 
of the pipe or rod extend above the 
ground to accommodate the target used in 
the horizontal position surveys. Mini- 
mizing the height of the monuments above 
the ground decreases the error in the 
horizontal position that results as the 
monuments tilt during subsidence. The 
1-inch rods have two advantages over the 
1-1/2-inch pipes. They are easier to 
drive into rocky ground to the required 
depth, and material costs for 6-foot-long 
monuments were $8.80 for the pipe and 
only $4.60 for the 1-inch rods. 

Monument Installation and 
Survey Schedule 

In the fall of 1978, monuments were 
installed over the first half of the 
first longwall panel (5E), and remote 
control points were located. The ini- 
tial traverse survey, performed in 
October prior to mining, provided the 



monument baseline elevations 
nate positions. 



and coordi- 



The second traverse survey was per- 
formed in July 1979 when approximately 
700 feet of the panel had been mined. At 
this time, no subsidence was detected 
over the mined-out area of the first 
panel. 

The second half of the first panel was 
monumented and the entire network sur- 
veyed in early September 1979. This pro- 
vided initial monument positions for the 
second half of the network, as well as 
the third survey of the first half of the 
panel, which, by the end of September, 
had retreated 1,500 feet. 

In September the decision was made 
to extend the monitoring program to 
the adjacent panels; however, early 
snowfalls made the site inaccessable 
and work over the second panel (6E) was 
delayed until the following spring. At 
this time (September), only 10 points had 
been located over the second panel. It 
was projected that panel 5E would be 
completed in another 8 months, based on 
the face advance per month for the first 
3 months of mining; however, the face 



16 




FIGURE 13. = Monument installation with a gas-powered hammer. 



17 



advance increased by an average of 150 
feet per month, and panel 5E was com- 
pleted in 5 months. When the site again 
became accessible in 1980, the second 
panel had retreated 1,200 feet. 

A complete survey on the first two 
panels, run in early July 1980, indicated 
that subsidence had progressed down the 
first panel during the winter months. In 
1980, monuments were installed over 
panels 7E and 8E (fig. 2), and the base- 
line positions were established. Five 
additional surveys, including two direct 
level surveys, were run on the first two 
networks at monthly intervals into early 
December. 

The site was accessible in 14 of the 28 
months from September 1978, when prelim- 
inary field work on the project began, 
through December 1980. Field work was 
performed at the site in 11 of the 14 
months, including 12 surveys on all or 
part of the monitoring network. Five of 
the twelve surveys were performed by the 
Bureau's contract surveyor during 1980; 
the remaining seven surveys were per- 
formed by Bureau personnel. 

All network layout and installation was 
performed by the Bureau and required 
a total of 375 worker-hours. This in- 
cluded reconnaissance, control point 
location, subsidence monument stakeout, 
and monument installation. A total of 
260 monuments have been installed over 
the four longwall panels. Additional 
monuments for direct strain measurements 
are being installed over the fourth panel 
during 1981. The seven traverse surveys 
performed by Bureau personnel involved 
205 worker-hours. (Hour totals do not 
include the travel time involved in 
reaching the remote study site.) 

Monitoring Procedures 

The Deer Creek subsidence monitoring 
program was designed to measure both 
vertical and horizontal movement of the 
subsidence monuments. It was recognized 
that, although the initial and final 
monument elevations showing the final 
subsidence profile and the associated 



horizontal strains were of prime impor- 
tance, another objective of the study was 
to determine the timing and rate of sub- 
sidence development. This required sev- 
eral periodic surveys during the summer 
months when the site was accessible. 

Based on the project objectives and the 
time and personnel constraints, it was 
determined that traverse surveys would be 
run by Bureau personnel during the first 
year of monitoring (1979). These surveys 
would provide both horizontal and verti- 
cal monument positions and allow the re- 
quired number of surveys to be completed 
during the short field season. The Bu- 
reau was able to obtain a contract sur- 
veyor to perform approximately 50 percent 
of the surveys in 1980, and both traverse 
and direct level surveys were run on the 
monitoring networks. 

The traverse surveys were run using an 
electronic distance-measuring instrument 
(EDM) and a second-order, optical-reading 
theodolite with micrometer readings of 
1 second (fig. 14). The EDM accuracy is 
±0.02 foot +6ppm. For the traverse sur- 
veys, horizontal and vertical angles and 
slope distance were measured to each sub- 
sidence monument from instrument stations 
with known coordinates. The elevation 
and coordinates of the instrument sta- 
tions were established from stable points 
beyond the influence of mining. To in- 
sure the stability of instrument stations 
located on nonsubsiding ground and to 
accurately determine the position of in- 
strument stations located over the pan- 
els, a closed traverse survey was run 
through all instrument stations and con- 
trol points as part of each survey. 

To facilitate the surveying of the more 
than 250 subsidence monuments, a target 
mounting unit was built for use in per- 
forming the traverse surveys. This tar- 
get unit (fig. 15) , which holds a prism 
for distance measurement and a target for 
angle measurement, is clamped securely 
onto the subsidence pin and then leveled. 
The unit requires minimal setup time, is 
compact and lightweight, and provides a 
stable target at a constant height above 
the monument. The standard mounting stud 



18 




FIGURE 14. - Theodolite with distance meter. 



19 




O 



J I I L 



J Scale, cm 



FIGURE 15. - Target-prism unit. 

will accept any target assembly that may 
be required, as well as vertical exten- 
sions to improve visibility. 

Beginning in 1980 with contract sur- 
veys, two standard, direct level sur- 
veys were run on the monitoring networks. 
These surveys were run to third-order ac- 
curacy using an automatic, self-leveling 
level and a level rod with 0.01-foot 
graduations (fig. 16). The accuracy of 
elevations from direct level surveys is 
greater than that from traverse surveys, 
which compute elevations from vertical 
angles and slope distance. The third- 
order level surveys have a maximum 
allowable closure error of 0.05 foot 
times the square root of the length of 



the level line in miles (0.05/S), which 
resulted in a standard error calculated 
from several level surveys of 0.02 foot. 
The elevations computed from the traverse 
surveys yielded a standard error of 0.08 
foot. 

Although the direct level surveys 
produce more accurate results than the 
traverse surveys, consideration must be 
given to the substantial extra cost of 
running level surveys in addition to 
traverse surveys, which are required to 
obtain horizontal positions. Continued 
monitoring at the Deer Creek site will 
utilize both level and traverse surveys, 
with more emphasis placed on vertical 
displacement and thus the direct level 
surveys. 

Measured Surface Subsidence 

Longitudinal subsidence profiles for 
panels 5E and 6E are shown on figures 17 
and 18, respectively. Results of the 
last two surveys, run in October and 
December 1980, indicate that subsidence 
was still occurring over both of the 
panels. Therefore, the final subsidence 
profile for the first panel (5E) cannot 
be determined until the surveys are com- 
pleted in 1981. Similarly, the final 
profile for panel 6E will be determined 
in 1982. 

To date, the maximum subsidence mea- 
sured over the two panels was 2.7 feet, 
which occurred near the midpoint of the 
panel lengths and south of the 5E panel 
centerline toward the chain pillars 
between panels 5E and 6E. The maximum 
subsidence was approximately 27 percent 
of the extraction height. The subsidence 
contours as of December 1980 are shown on 
figure 19. The maximum subsidence mea- 
sured over the centerline of panel 5E was 
2.6 feet, while the maximum over panel 6E 
was 1.6 feet. Figure 17 shows a signifi- 
cant amount of subsidence occurring over 
panel 5E as panel 6E was mined during 
1980. This indicates that subsidence 
will continue over panel 6E and increase 
as the adjacent panel (7E) is mined dur- 
ing 1981. 



20 




FIGURE 16. - Level instrument and rod with 0.01-foot graduations. 



21 



9200 



9,100- 



<u 
a> 



< 
> 

LlI 

_l 



stooo 



> 8i900 



8.800- 



8,700 




045 040 035 020 015 

FIGURE 17. - Subsidence profiles— panel 5E. 



9,200 r 



% 9,I00|- 



8,800 



-Original ground 
elevation 




E40 E35 E20 EI5 ElO 

FIGURE 18. - Subsidence profiles-panel 6E. 



22 



C50 



PANEL 5E 
o o o o 



PANEL 6E 



49 



PANEL 7E 
47' 




FIGURE 19. - Subsidence contours. 



BMI 



•Dl 




L_L_JL 



500 
jj 



Scale, feet 



The two longwall panels completed to 
date have face lengths of 480 and 540 
feet. The total mined width for the 
first panel, including two 20-foot en- 
tries, is 520 feet, which represents an 
average width-to-depth ratio of 0.36. 
This ratio is less than that required 
for maximum subsidence and thus assures 
a subcritical condition; that is, the 
width of the opening is insufficient to 
allow the maximum possible subsidence to 
occur. A subcritical condition is char- 
acterized by a U-shaped subsidence pro- 
file; whereas, a supercritical condition 
results in a flat-bottomed subsidence 
profile, in which more than one point 
near the center of the panel reaches max- 
imum subsidence. 

A subsidence profile across the two 
panels is shown on figure 20. The total 
mined width across both panels, including 
entries and chain pillars, is approxi- 
mately 1,200 feet, which represents a 
width-to-depth ratio of 0.80. This total 
width also produces the characteristic 
subcritical U-shaped subsidence profile. 
At this point in the subsidence develop- 
ment, there was no evidence in the sub- 
sidence profile of the two rows of chain 
pillars between the two mined panels. 



»,^UU' 


— 




^Original ground elevation 






July 1980- 


^ ■ 


^^^ 


^ 9,100 

0) 

<u 

«.- 


^ 


^=^ 


^^^August^^^ 


'^^^^^ 


- 


\ 


^ ^S^September 


^r^ 7^ 


■ — ■ 


9,000 

1- 




>^ >^980 


^ecemberX 
__I980X 




n\ 8,900 


— 


Maximum ^w 
subsidence, ^^^^ 
_^7 feet^^ 


^ 




R Rnn 




PANEL 5E 
1 1 


PANEL 6E " 



PIS 



PIO 



P5 



O 

z 

UJ 

to 



-3 



PI 



FIGURE 20. - Subsidence profiles across panels 
5E and6E. 

Because subsidence was not complete 
over the west end of the panels at the 
time of the last survey, the east end of 
the first panel is the only location at 
which an angle of draw can be calculated 
at this time. As monitoring continues, 
there will be a minimum of 10 separate 
locations at which the angle of draw 
will be measured. The calculated angle 
of draw for the east end of panel 5E is 
27 degrees from vertical. This figure 
is based on elevations from traverse 
surveys and will reflect the accuracy of 
this survey method. The angle of draw. 



23 



determined by the limit of subsidence 
beyond the panel, is extremely sensitive 
to surveying error; therefore, several 
measurements, including some with greater 
accuracy, will be required before the 
angle of draw for the Deer Creek site can 
be confidently stated. In addition, a 
major fault occurs beyond the east end of 
the panels in the area affected by the 
angle of draw. This fault could tend to 
decrease the angle of draw and may 
account for the 0.7-foot step in the sub- 
sidence profile between two adjacent 
points approximately 150 feet east of the 
panel. Continued measurements will also 
clarify the effect, if any, of this fault 
on the angle of draw. 

Subsidence Development 

The first indication of subsidence over 
panel 5E occurred in the September 1979 
survey (fig. 17). When monument eleva- 
tions were compared with those from the 
initial survey, the first 1,400 feet of 
the panel showed approximately 0.25 foot 
of subsidence. At this time, the long- 
wall face had retreated 350 feet beyond 
the last point of measured subsidence. 
The July 1979 survey had shown no sub- 
sidence over this panel, which indicates 
that the initial subsidence occurred be- 
tween July and September 1979. At the 
midpoint of this time span, in August, 
the face had advanced 1,050 feet. At a 
minimum, the face had advanced 550 feet 
(as of July) before any subsidence was 
measured at the surface. 

Following the September 1979 survey, 
the site became inaccessible until July, 
because of heavy snowfalls. Mining of 
the first panel (5E) was completed in 
December 1979. The next survey was per- 
formed on July 9, 1980. At this time, 
the subsidence over panel 5E had in- 
creased to a maximum of 1.6 feet and had 
progressed down the entire length of the 
panel. The adjacent panel (6E) had re- 
treated approximately 1,400 feet at the 
time of this survey. 

Between July and December 1980, subsid- 
ence continued over panel 5E, increasing 
in magnitude in the direction of mining. 



During November 1980, there was no addi- 
tional subsidence over the first 700 feet 
of panel 5E; however, continued settling 
of up to 0.4 foot occurred over the re- 
mainder of the panel. 

Between July and December 1980, subsid- 
ence over panel 6E reached a maximum of 
1.6 feet near the midpoint of the panel 
length (fig. 18). As with panel 5E, 
there was no additional subsidence over 
the first 700 feet of panel 6E during 
November 1980. Up to 0.5 foot of subsid- 
ence occurred during November over the 
second half of panel 6E, at the same 
position on the panel length as the 0.4 
foot measured over panel 5E and mentioned 
above. Panel 6E was completed in Decem- 
ber 1980, and third panel (7E) mining 
began in February 1981. 

Data Processing 

All calculations and much of the plot- 
ting from the subsidence surveys are per- 
formed by computer. Although the calcu- 
lation of coordinates and evaluations 
from field survey notes is not complex, 
the large number of points included in 
each survey, along with the number of 
surveys to be performed over the duration 
of the study, makes computer calculation 
and data storage advantageous. 

Field data from the subsidence surveys 
are typed into a computer file, printed 
out, and then compared with the field 
notes so that obvious errors can be 
edited prior to computing position coor- 
dinates for the subsidence points. The 
raw survey data for the traverse surveys 
entered into the computer file consist of 
station names, horizontal and vertical 
angles, slope distances, and the target 
and instrument heights. Before the sub- 
sidence point coordinates are computed, 
the instrument station positions are 
checked and adjusted if necessary. The 
northing, easting, and elevation of each 
subsidence monument is then computed and 
stored in a data file representing that 
particular survey. This information can 
be readily accessed and used as input for 
programs that perform calculations such 
as coordinate or elevation differences 



24 



between any two surveys. The coordinate 
data as well as the results of any calcu- 
lations can be printed out or plotted 
depending on the nature of the results 
and the intended use. In addition, the 
raw input data from the field books are 
held in storage and can be printed out 
anytime questions arise involving the 
field data. If changes are required, the 
data can be edited and rerun to produce 
the corrected coordinate data. 



Data input for the direct level surveys 
consists of station names and rod read- 
ings. The computer program calculates 
point elevations and closure errors and 
then adjusts the monument elevations 
accordingly. Resulting elevations are 
stored on file for access by other pro- 
grams in the same manner as the results 
of the traverse surveys. The input data 
are also stored and can be recalled for 
checks and editing. 



CONCLUSIONS 



Planned subsidence monitoring over the 
Deer Creek Mine includes the surface 
area over four adjacent longwall panels. 
To date, only two panels have been mined; 
therefore, results presented herein 
are preliminary to the final subsidence 
conditions. 

The maximum subsidence measured over 
the two mined longwall panels was 2.7 
feet as of December 1980, some 27 percent 
of the extracted seam height. Subsidence 
was continuing over both panels at this 
time; therefore, the final subsidence 
profiles could not be determined. The 
preliminary estimate of the angle of draw 
is 27 degrees. 

An appreciable impact of adjacent panel 
mining on the final subsidence profiles 
over previously mined panels has been es- 
tablished. The time lag between initial 



mining and measura 
with the time dur 
continues to occur, 
cent panel is mined 
over the previous 
This interaction p 
definition of subs 
single panel. 



ble subsidence, along 

ing which subsidence 

means that the adja- 

before the subsidence 

panel is complete. 

recludes isolation or 

idence from mining a 



Topography is not expected to influ- 
ence the subsidence significantly because 
of the rolling nature of the terrain, 
with no abrupt changes in the overbur- 
den thickness relative to the depth of 
mining. 

Through December 1980, there was no 
evidence of surface damage from subsid- 
ence over the two mined panels. No 
cracking or downwarping of the surface 
was visible during any of the surveys. 



INT.-BU.OF MINES,PGH.,P A. 26384 



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